The present invention relates to a hydraulic elevator with which the energy efficiency on operation is enhanced. Moreover, the invention is concerned with an improved control method of an elevator, particularly, with a speed control method of a hydraulic elevator using an inverter power source for a more comfortable ride in the elevator.
Among conventional hydraulic elevators are those shown in FIGS. 2 and 3. In the elevator shown in FIG. 2, a ram (15) provided with a cage (2) for carrying persons and/or burdens at its upper end is inserted into a hydraulic cylinder (16) and the working fluid (X) is flown-in from an oil tank to the hydraulic cylinder (16) or flown-out from the hydraulic cylinder (16) to the oil tank by a hydraulic pump not shown in the diagram to move the cage (2) up or down. In the elevator shown in FIG. 3, the upper end of a cage (2) for carrying persons and/or burdens is attached to one end of wire (17) and a weight (19) is attached to the other end through pulleys (18) to pull the cage (2) upward by the weight of weight (19) in the case of ascending cage (2), thus to alleviate the load on a hydraulic pump by a portion of pressure corresponding to the tension, or the like.
In the case of the hydraulic elevator shown in FIG. 2, however, the power factor, that is, the required power of hydraulic pump-driving motor when ascending said elevator at a speed V can be expressed by the following formula, where A is the weight of cage (2), B is the weight of ram (15) and W is the maximum burden weight. EQU (A+B+W).times.V (1)
Since overall load is applied as it is as a load in this way, a large-capacity motor is required and further the temperature rise of working fluid is also significant.
Moreover, in the case of hydraulic elevator shown in FIG. 3, required power when ascending said elevator at a speed V is expressed by the following formula, where Z is the weight of weight (19). EQU (A+B+W-Z).times.V (2)
The load of the motor in formula (2) becomes lower over the formula (1) permitting the use of a motor with a relatively small capacity. However, since weight (19) is retained by the building, the structure of the building must be of a large scale or include strong reinforcement.
Furthermore, when descending these hydraulic elevators, it is common to construct a circuit allowing working fluid from the cylinder to pass through a throttle valve and to return to the oil tank under control of speed by the self-weight of elevator. In the case of hydraulic elevator shown in FIG. 3, however, this works effectively only when the weight (Z) of weight (19) is lighter than the total weight (W +A+B) of burden weight, cage and ram.
As a result of extensive investigations in view of this situation, an energy conservation type hydraulic elevator having solved these problems has been developed by the invention.
Moreover, in a conventional hydraulic elevator, a three-phase induction motor (hereinafter referred to as motor) was combined with the hydraulic pump and the working fluid was transported from the oil tank to the cylinder by the hydraulic pump and conversely from the cylinder to the oil tank by a hydraulic directional control valve to allow the elevator to move up and down. In this method, the speed control of the hydraulic elevator was performed by directionally controlling the flow rate of working fluid with a pilot directional control valve. With such a control method, however, there is room for improvement because the elevator is uncomfortable when it starts to descend, the elevator vibrates significantly on acceleration, and further the temperature rise of working fluid also becomes high.
Hence, a method of controlling the speed of elevator, wherein the number of revolutions of a motor is changed by changing the frequency of power source to the motor using an inverter control power source, and others have been adopted so far, but the problems aforementioned could not be said to have been enough solved.
As a result of extensive investigations in view of this situation, a speed control method has been developed by the invention, wherein a highly smooth, comfortable ride can be achieved without giving a large shock to the elevator upon starting descent of the elevator and wherein the comfortableness on descending arrival at the floor is improved.